libstdc++: Fix --enable-cstdio=stdio_pure [PR110574]
When configured with --enable-cstdio=stdio_pure we need to consistently
use fseek and not mix seeks on the file descriptor with reads and writes
on the FILE stream.
There are also a number of bugs related to error handling and return
values, because fread and fwrite return 0 on error, not -1, and fseek
returns 0 on success, not the file offset.
libstdc++-v3/ChangeLog:
PR libstdc++/110574
* acinclude.m4 (GLIBCXX_CHECK_LFS): Check for fseeko and ftello
and define _GLIBCXX_USE_FSEEKO_FTELLO.
* config.h.in: Regenerate.
* configure: Regenerate.
* config/io/basic_file_stdio.cc (xwrite) [_GLIBCXX_USE_STDIO_PURE]:
Check for fwrite error correctly.
(__basic_file<char>::xsgetn) [_GLIBCXX_USE_STDIO_PURE]: Check for
fread error correctly.
(get_file_offset): New function.
(__basic_file<char>::seekoff) [_GLIBCXX_USE_STDIO_PURE]: Use
fseeko if available. Use get_file_offset instead of return value
of fseek.
(__basic_file<char>::showmanyc): Use get_file_offset.